1. Field of the Invention
The present invention relates to an electric power steering apparatus for an automobile. More particularly, the present invention relates to an electric power steering apparatus for an automobile having a support member and an elastic member for applying elastic force in the axial and radial directions of a worm shaft so that the gap between a worm wheel and the worm shaft is compensated for and backlash is avoided.
2. Description of the Prior Art
As generally known in the art, power steering apparatuses for automobiles include a hydraulic power steering apparatus utilizing hydraulic pressure of a hydraulic pump, which has been used since its initial introduction, and an electric power steering apparatus utilizing an electric motor, use of which has been gradually universalized since the 1990's.
In the existing hydraulic power steering apparatus, a hydraulic pump, which is a power source for supplying steering power, is driven by an engine, which causes the hydraulic pump to continuously consume energy regardless of whether or not the steering wheel is being rotated. In the electric power steering apparatus, when steering torque is generated by rotation of a steering wheel, a motor supplies steering power in proportion to the generated steering torque. Therefore, in terms of energy efficiency, the electric power steering apparatus is more advantageous than the hydraulic power steering apparatus.
FIG. 1 shows the construction of a conventional electric power steering apparatus for an automobile, and FIG. 2 is a sectional view showing a reduction mechanism according to the prior art.
As shown in FIGS. 1 and 2, a conventional electric power steering apparatus includes a steering system 100, which includes elements leading from a steering wheel 101 to both wheels 108, and a steering power mechanism 120 for supplying steering power to the steering system 100.
The steering system 100 includes a steering shaft 102 having an upper end connected to the steering wheel 101 and a lower end connected to a pinion shaft 104 via a pair of universal joints 103, so that the steering shaft 102 rotates together with the steering wheel 101. The pinion shaft 104 is connected to a rack bar 109 via a rack-pinion mechanism 105. Both ends of the rack bar 109 are connected to the wheels 108 of the automobile via tie rods 106 and knuckle arms 107.
The rack-pinion mechanism 105 includes a pinion gear 111 formed on the lower end of the pinion shaft 104 and a rack gear 112 formed on one side of the outer peripheral surface of the rack bar 109 to engage with the pinion gear 111. The rack-pinion mechanism 105 converts the rotational motion of the pinion shaft 104 into a linear motion of the rack bar 109. Particularly, when the driver operates the steering wheel 101, the pinion shaft 104 rotates accordingly. The rotation of the pinion shaft 104 causes the rack bar 109 to move linearly in the shaft direction. The linear motion of the rack bar 109 is transmitted to and thereby operates the wheels 108 via the tie rods 106 and the knuckle arms 107.
The steering power mechanism 120 includes a torque sensor 121 for sensing steering torque applied to the steering wheel 101 by the driver and outputting an electric signal in proportion to the sensed steering torque, an ECU (electronic control unit) 123 for generating a control signal based on the electric signal from the torque sensor 121, a motor 130 for generating steering power based on the control signal from the ECU 123, and a reduction mechanism 140 having a worm 201 and a worm wheel 203 for transmitting the steering power from the motor 130 to the steering shaft 102.
The reduction mechanism 140 includes a worm shaft 210, both ends of which are rotatably supported by bearings 205 and 206, respectively. The worm 201 is positioned on one side of the outer peripheral surface of the worm shaft 210. The worm 201 is adapted to engage with the worm wheel 203, which is formed in a predetermined position on the outer peripheral surface of the steering shaft 102. The worm shaft 210 is driven by the motor 130.
The electric power steering apparatus is operated as follows: when the driving wheel 101 is rotated, driving torque is generated and transmitted to the rack bar 109 via the rack-pinion mechanism 105. In addition, the generated steering torque causes the motor 130 to generate steering power, which is transmitted to the rack bar 109. As such, the steering torque generated by the steering system 100 is combined with the steering power generated by the motor 130, so that the rack bar 109 is moved in the shaft direction.
However, conventional reduction mechanisms have a problem in that, as the automobile is operated and steered over a period of time, the teeth of the worm 201 and worm wheel 203 may be worn down. In addition, the worm wheel 203 may contract in the winter season, because it is made of a synthetic resin in most cases.
When the teeth of the worm 201 or the worm wheel 203 wear down or when the worm wheel 203 contracts, a gap is generated between the teeth of the worm 201 and the teeth of the worm wheels 203. As a result, backlash occurs in the region where the teeth of the worm 201 engage with the teeth of the worm wheel 203.
In addition, when conventional reduction mechanisms are subjected to inverse pressure, which is caused by kickback, for example, and resulting excessive torque, gapped portions of bearings on both ends of the worm shaft 210 may generate noise.
In an attempt to solve these problems, two elastic bodies are inserted, according to the prior art, in such a manner that elastic force is applied in the radial and axial directions of the worm shaft, respectively. However, this approach increases the number of components and lengthens the manufacturing process.